They contacted more than 3,000 randomly selected households between January and March this year and asked about displacement, infrastructure loss and causes of death.

Were deaths preventable?

Aleem Maqbool, BBC News, Washington

Many Puerto Ricans we spoke to felt their immense suffering after the hurricane had been trivialised and that the emergency response has been lacklustre.

A relatively small number of people may have been killed by the physical impact of the storm, but six months later we met people who had lost relatives as a result of interrupted medical care and saw others struggling to pay for expensive generators on which they were running vital life support equipment.

There was also reported to have been a spike in the number of suicides. We found many still without homes and thousands who had been living without electricity since the day Hurricane Maria struck.

Whatever the true number of those killed as a result of the storm, it is now certain to be many times the official figure. The question for US authorities about Puerto Rico, an American territory, is how many of those deaths could have been prevented with a better emergency response?

They then compared their results with the official mortality rates for the same period in 2016, more than a year before the hurricane struck the island.

The researchers said that interrupted medical care was the "primary cause of sustained high mortality rates in the months after the hurricane".

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Media captionSix months after hurricanes, many on this US island still suffer in the dark

Disruption to health care was a "growing contributor to both morbidity and mortality" in natural disasters, they said, because growing numbers of patients had chronic diseases and used sophisticated equipment that relied on electricity.

On Twitter, several users noted that the death toll exceeds the nearly 3,000 Americans that were killed during the 9/11 attacks.